So... I kinda started noticing the quest book lag...
And it made me think, since there is no lag on the start page, no lag on the quest detail, only when you can select chapters or the quests inside the chapters! And, without looking at code I can think of one reason why this could be happening. Quest availability checking!
The huge number of quests could not be the problem since other packs have same or higher number but the number of dependencies on the quests could be a problem. And it could be being magnified by the reputation system. Probably, when the quest book is open, it is always checking if all the dependencies for a quest are made at every tick. When you only have a few quests available, this is a fast process since the processing stops down the tree as soon as you haven't the requirements necessary. But once you start getting a lot of open quests, the tree gets bigger and takes more time to check. With reputation and many quests dependencies from other chapters, not only gets bigger as its also more complex and takes more time to check! And this could be even worst if for some reason, the system checks for availability on all quests in the book and not only the ones in the current chapter and their dependencies.
One fast test, just remove all or most of the dependencies on the quests and see if still lags.